1. Field of the Invention
The present invention relates to a system for manufacturing a semiconductor device, and more specifically, it relates to a system for forming a thin film on a surface of a sample or etching the surface of the sample with plasma.
2. Description of the Prior Art
FIG. 12 illustrates the structure of a plasma system described in Japanese Patent Laying-Open No. 7-263353 (1995). As shown in FIG. 12, the plasma system includes a reaction chamber 1, a stage 12 for receiving a sample 11 thereon, a pulse gas valve 20, a gas introduction tube 141, a pressure detector 142, a pressure controller 143 and a pressure regulator 144.
The gas introduction tube 141 supplies a gas from a gas cylinder (not shown) into the pulse gas valve 20. The pressure detector 142 and the pressure regulator 144 are connected to intermediate portions of the gas introduction tube 141. The pressure controller 143 drives the pressure regulator 144 on the basis of a signal from the pressure detector 142.
The gas introduced from the gas introduction tube 141 is supplied into the pulse gas valve 20 and pulsatively introduced into the reaction chamber 1. The pressure detector 142 sequentially detects the pressure in the gas introduction tube 141 and feeds back the same to the pressure controller 143. The pressure controller 143 controls the pressure regulator 144 for maintaining the pressure in the gas introduction tube 141 at a prescribed value.
Even if the back pressure of the pulse gas valve 20 fluctuates, therefore, the flow rate of an etching gas introduced into the reaction chamber 1 can be kept under prescribed conditions for maintaining the pressure in the reaction chamber 1 at a prescribed value.
However, the aforementioned conventional plasma system has the following problems:
In the aforementioned gas supply system, the flow rate of the gas supplied from the pulse gas valve 20 into the reaction chamber 1 is univocally determined by the pressure at the inlet of the pulse gas valve 20. In order to stably pulsatively supply the gas at a prescribed flow rate, therefore, the pressure at the inlet of the pulse gas valve 20 must be maintained constant through the pressure controller 143. The pressure controller 143 must be employed also for changing the gas flow rate. However, the gas flow rate cannot be correctly controlled or finely regulated through the pressure controller 143.
Further, the flow rate of the gas supplied into the reaction chamber 1, which is controlled by the pressure controller 143, must be calculated from the pressure in the reaction chamber 1. Therefore, the correct gas flow rate cannot be immediately recognized.
The pressure controller 143 controlling the pressure through the feedback signal from the pressure detector 142 is effective for slow pressure change. However, the pressure controller 143 cannot cope with abrupt pressure change, and hence it is difficult to maintain the gas flow rate at a constant value when remarkable pressure change takes place.
In case of supplying a gaseous mixture into the reaction chamber 1 through the single pulse gas valve 20, the gas mixing ratio (flow ratio partial pressure ratio) is determined through the ratios of the specific heat of the gases and the pressure at the inlet of the pulse gas valve 20. When the difference between the ratios of specific heat of the gases or the pressure difference between the gases is remarkable, therefore, it is difficult to obtain a desired mixing ratio.
In case of employing a plurality of pulse gas valves 20 or exchanging the pulse gas valve 20, gases are supplied at different flow rates even if the valves 20 are pulsatively driven under the same conditions, due to the individual difference between the opening degrees thereof. It is difficult to obtain a desired gas flow rate also in this case.
When supplying a gas into a plurality of reaction chambers 1 from a single gas cylinder and starting processing in one of the reaction chambers 1 during processing in another reaction chamber 1, the pressure in a pipe temporarily fluctuates to change the flow rate of the gas supplied into the reaction chambers 1. It is difficult to obtain a desired gas flow rate also in this case.